Superconducting electrical conductors

ABSTRACT

An electrical conductor comprising filaments of superconductor in a non-superconductor matrix, each filament having a maximum thickness of about 0.005 cm and being twisted or transposed about the other filaments in accordance with the given formula, and methods of making the same.

O United States Patent [151 3,662,093 Wilson et al. 1 May 9 1972 [54] SUPERCONDUCTING ELECTRICAL [56] References Cited CONDUCTORS UNITED STATES PATENTS [72] Inventors: Martin Norman Wilson, Abingdon; Peter 3,029,496 4/1962 Levi ..29 599 UX Newbury; 3 306 972 2/1967 Laverick 33 5/216 x Walters Abingdon; John David Lewin, Oxford; Robert Walter Broomfield, Street- FOREIGN PATENTS OR APPLICATIONS 1e Sutton Coldfield; Robert Lesli Gz Four O Sutton c fi Great Britain l 6 of England 1,096,535 12/1967 Great Britain ..174/D1G. 6

[73] Assignees: Science Research Council, London; lm- OTHER PUBLICATIONS i Metal Industries (Kynoch) Cline H. E. Superconductivity of Composite of Fine Niobi- Blrmmgham' England um Wire in Copper in Journal of Applied Physics Vol. 37 [22] Filed: Apr. 1, 1969 No. 1 January 1966 pg. 5 pg. 6 left col. relied on. [21] Appl- N .2 81 ,0 Primary Examiner- E. A. Goldberg Attorney-Cushman, Darby & Cushman [30] Foreign Application Priority Data [57] ABSTRACT Apr. 3, 1968 Great Britain ..16,023/68 An electrical conductor comprising filaments of supercondao tor in a non-superconductor matrix, each filament having a [52] U.S. C1. ..174/128, 174/114, 174/DIG. 6 maximum thickness of about 0.005 cm and being twisted or [51] 11/00 transposed about the other filaments in accordance with the [58] Field of Search ..174/90, 128, 1 14; 29/199, 599; given formu'a and methods of making the Same.

' SUPERCONDUCTOR 17 Claims, 7 Drawing Figures PATENTEDMY 9 .97?

mm 1 of 2D 1 1 S /V n o o ALLOY CONTENT 1 FIG. I

PATENTEDMM 9 I972 SHEET 2 [1F 6 FIG 2.

FIG.

PATENTEDMAY 9 m2 I 3,662,093

SHEET 4 [1F 6 /CUPRONICKEL g SUPERCONDUCTOR SHEET 5 BF 6 PATENTED MAY 9 I972 EQZOEB PATENTEDMM 91912 3,662,093

SHEET 5 [IF 6 AMPERES AMPERE This invention relates to electrical conductors which have superconducting properties at cryogenic temperatures, i.e. at

temperatures of about 42 K. The invention is also concerned with methods of manufacturing such conductors.

BACKGROUND OF THE INVENTION Electrical conductors having superconducting properties at cryogenic temperatures have been proposed, in which a plurality of filaments of superconductor material are embedded in a stabilizing matrix of copper or other ductile material which is not superconducting at those temperatures and has high coefficients of electrical and thermal conductivity at cryogenic temperatures, eg high purity high conductivity copper or aluminum. In this connection, high conductivity copper has an electrical resistivity of only 0.01-0.002 microohm-cm. at 42 K., and the electrical resistivity of aluminum is very similar to this value.

The purposes of the stabilizing matrix are to absorb and conduct away the heat evolved both during jumps of flux penetration into the superconductor material during changes in magnetic field and electrical current conditions, and the heat resulting from mechanical movement of the conductor, and to provide a low resistance electrical shunt to enable current temporarily to by-pass any region of superconductor material which becomes non-superconducting because of the any temperature increase, particularly arising from such a flux jump. When sufficient heat has been conducted away from such a region, the return of that region to superconducting properties enables the current to pass once more through the superconductor material alone.

When substantial lengths of these conductors are wound into coils, they usually attain much lower field strengths than those calculated from tests on short lengths of the conductor. The degradation in performance obtained when coils are wound from the electrical conductors is known as coil degradation.

Coil degradation can be overcome by the use of even larger amounts of the stabilizing material, and sophisticated cooling provisions, but the end result is a coil which, although it is not degraded, is expensive to build and is larger than necessary because of its low overall current density. In addition the low overall current density limits the range of application of such coils.

Coil degradation arises principally from instabilities in the superconductor known as flux jumps. These occur when the closed loops of current induced in the superconductor material by the magnetic field become unstable and break down with evolution of heat. Therefore, there is a need for a superconductor in which flux jumps are eliminated or are reduced to such a degree that portions of the superconductor are not changed from the superconducting to the non-superconducting state, whereby the stabilizing matrix material could theoretically be eliminated.

Following this need, it has been proposed in a Culham Laboratory Report, reference CLM-Pl2l (1966) that a single filament of superconductor material, which has a filament thickness perpendicular to the field to which the filament is to be subjected less than a critical size, typically about 0.005 cm. for typical current densities of 2-5 X amps/sq.cm. would be free from flux jumps; closed loops of current would still be formed in the material, but these would be stable and would not usually break down with sudden localized evolution of heat. Furthermore the magnetic energy associated with these loops would be reduced to such an extent that even if breakdown were to occur, the heat released would be insufficient to drive the material into a non-superconducting state. The actual maximum size for the filament thickness needs to be determined by experiment, and in particular will vary with the current density, the variation of current density with temperature, and the thermal capacity per unit volume of the superconductor material. (See also l-lancox, Physics Letters Vol. 16, pg. 208 (1965), Swartz and Bean, Bulletin American Physical Society" Vol. 10, pg. 359 (1965), Neuringer and Shapira, Physical Review Vol. 148, pg. 231 (1966), Wipf and Lubell, Physics Letters Vol. 16, pg. 103 (1965), Y.B. Kim, Proceedings of Cryogenic Engineering Conference (Tokyo)", pg. 168 (April 1967), and Stability of Flux Motion in Superconducting Coils by Hancox in Proceedings of Tenth Conference on Low Temperature Physics" (Moscow 1966).)

In practice, filaments of such a thickness, which would normally be their diameter, would be very difficult to deal with individually, so that a composite of a plurality of superconductor filaments held in a matrix, so as to produce a conductor of reasonable size, would be necessary for winding magnetic coils. Thus, a composite superconductor has been proposed in which the filaments are embedded in a matrix of copper, or an insulator (to reduce coil inductance), but the filaments will normally be in electrical contact with one another, even if this only takes place at the ends of the composite conductor. The result of this interconnection is that adjacent lengths of filaments are electrically connected to one another, and hence they form a closed electrical path which will be subjected to changes in magnetic flux linked through the path, and the resulting magnetization currents in those lengths will mean that the composite will usually act in a somewhat similar way as a solid superconductor wire of the same overall diameter as that of the composite. Hence, the limit of 0.005 cm. filament diameter exemplified above will no longer be adhered to.

OBJECT OF THE INVENTION Thus, it is an object of the invention to provide an electrical conductor comprising a plurality of filaments of superconductor material each having a maximum thickness of about 0.005 cm. in which the filaments are effectively not in electrical contact with one another so that flux jumps are at least minimized.

SUMMARY OF THE INVENTION In accordance with the invention an electrical conductor comprises a plurality of filaments of superconductor material contained within and separated from one another by a matrix of at least one non-superconductor material, each filament having a maximum thickness of about 0.005 cm. and being twisted or transposed about the other filaments with a length L along the conductor between each return of that filament to a given angular position relative to the matrix given by the relationship:

where k is a space factor equal to the ratio between the linear dimensions of superconductor and those of superconductor plus matrix materials in a direction parallel to the magnetic field to which the conductor is to be subjected, J is the critical current density in amps/cm. of the superconductor material (in zero or very low magnetic fields), d is the average thickness in cms of each filament perpendicular to the magnetic field, p is the electrical resistivity in ohm-cms of the nonsuperconductor material separating the filaments from each other and having the highest electrical resistance, B is the rate of change of field in gauss/second to which the electrical conductor is to be subjected, and a is a number which is greater than 3 and is sufficiently large for the variation in flux rate of change along the length L divided by the mean flux rate of change B along the length L to be less than the thickness d di vided by the thickness of the conductor perpendicular to the field.

In accordance with the invention also a method of manufacturing an electrical conductor comprises taking a plurality of filaments of superconductor material, providing them with a matrix of at least one non-superconductor material, and the matrix separating the filaments from each other, and twisting or transposing each filament about the other filaments with a distance L along the conductor between each return of that filament to a given angular position relative to the matrix given by the relationship:

L M 21 X 1 where k is a space factor equal to the ratio between the linear dimensions of superconductor and those of superconductor plus matrix materials in a direction parallel to the magnetic field to which the conductor is to be subjected, J is the maximum current density in amps/cm to be carried by the superconductor material (in zero or very low magnetic fields), d is the average thickness in cms of each filament perpendicular to the magnetic field, p is the electrical resistivity in ohm-cm. of the non-superconductor material separating the filaments from each other and having the highest electrical resistance, B is the rate of change of field in gauss/second to which the electrical conductor is to be subjected, and a is a number which is greater than 3 a nd is sufficiently large for the variation in flux rate of change B along the length L divided by the mean flux rate of change B along the length L to be less than the thickness d divided by the thickness of the conductor perpendicular to the field.

Preferably the number a is greater than 10.

The relationship given above is derived from a consideration of the magnetization current induced along a length L of a pair of parallel superconducting filaments and through the intervening matrix by a rate of change of flux B, this current being the critical current when a equals 1. Hence with a twist or transposing length L as given by the relationship with a equal to or less than 1, the conductor would act approximately as a solid superconductor wire and would then be unstable. Increase of a to 3 reduces the voltage inducing the magnetization current passing through the matrix by a factor of 3, and approximately triples the electrical resistance between the superconducting filaments to reduce magnetization currents through the matrix by a factor of about 10. This will greatly increase stability, and further improvement will be effected by increasing the value of a to a greater extent.

A value for a of greater than 3 will not always be satisfactory, particularly for irregular arrays of superconducting filaments, and for large conductors. The minimum value ofa varies with the square root of the quotient between the conductor thickness perpendicular to the field and the filament thickness d.

When the distance S (cm.) between filaments is small compared with the filament thickness d (cm.) the value ofL calculated by the preceding formula must be multiplied by the approximate correction factor (S/S d)" If the matrix separating the filaments is composed of more than one material, the values of p and S to be used in the preceding formula are those corresponding to the material which has the highest value ofp.

When p is high, corresponding to good insulation between the filaments, the relationship given above gives a high value for L. However, the non-uniformity of field throughout the coil may result in a build-up of magnetization currents in the matrix unless a certain maximum length L is used, this being given by a being sufficiently large for the variation in flux rate of change Q along the length L divided by the mean flux rate of change B along the length L to be less than the thickness d divided by the thickness of the conductor perpendicular to the field.

The matrix is preferably a ductile material to enable the superconductor and non-superconductor materials to be worked together to produce the required physical dimensions. Also, because the length L increases with the resistivity p of the nonsuperconductor material, for practicability the resistivity p needs to be at a substantial level.

Thus preferably the matrix is of a ductile copper-base alloy which contains not less than 50 weight/cent copper and has an electrical resistivity of at least 6 micro-ohm-cm. at 20 C.

it is the electrical resistivity at the operating temperature of the electrical conductor which is of paramount importance, i.e. usually at 4.2 K., but this is usually only i or 2 micro-ohmcm. less than the resistivity at 20 C. so that, for ease of experimental verification, it is the minimum resistivity at 20 C. which has been specified.

The copper-base alloy may contain the alloying additions up to 50 weight/cent nickel, up to 30 weight/cent manganese, up to 40 weight/cent zinc, up to 8 weight/cent tin, up to 10 weight/cent aluminum, up to 4 weight/cent silicon, up to 2 weight/cent iron, up to 2 weight/cent chromium and up to l weight/cent phosphorus.

Preferably the ductile copper-base alloy contains 5-50 weight/cent nickel, 0-2 weight/cent manganese, balance copper, which produces an electrical resistance for that range of alloys of at least 7 micro-ohm-cm. at 4.2 K., whereby a high degree of electrical insulation is achieved between the superconductor filaments, so that the length L can be substantial, although the ductility of the alloy enables the matrix and the filaments to be co-processed. In this way the electrical conductor can be manufactured using a co-processingroute. Also these alloys have hardnesses which can be very similar to selected superconductor materials. Matching of the hardnesses of the matrix and the superconductor materials facilitates co-processing, enables co-processing to be continued to such a degree that very fine filaments of superconductor material are attained without breakage, and enables twisting to be carried out without breakage of the filaments. In this way there can be achieved substantial lengths of homogeneous electrical conductor, the superconductor filaments being continuous throughout the length of the conductor.

The length L, as mentioned above, increases with the resistivity p of the non-superconductor matrix material. Thus the resistivity p is preferably as high as possible and the present invention contemplates a highly resistive matrix whereby there is produced a large L. However, the value of L has a limit determined by the value of a being sufficiently large for the variation in flux rate of change along the length L divided by the mean flux rate of change B along the length L to be less than the thickness d divided by the thickness of the conductor perpendicular to the field. However, to enable the superconductor and non-superconductor materials to be co-worked, a ductile matrix is preferred, in which case a non-superconductor material is chosen which has a high resistivity, for example the cupro-nickel alloys mentioned above.

A further factor involved is the critical current density J in the superconductor material; this is dictated byv the selection of superconductor made. There is also the space factor k, which is equal to the ratio between the linear dimensions of superconductor and those of superconductor plus matrix materials in a direction parallel to the field to which the conductor is to be subjected. A further factor is the thickness d of the superconductor filaments; this has the limited maximum value described above prescribed by the requirement that any flux jumps in each superconductor filament will not increase the temperature of the filament above that at which the conductor can carry the required transport current. in practice a maximum filament diameter is about 0.005 cm. The fourth factor is B which is arranged to be as low as possible, consistent with a reasonable rate of change of the field and possib le variations in the power supply to the conductor. A typical B is 1,000 gauss per second.

Preferably the superconductor material is a superconducting niobium-titanium alloy, for example niobium 44 weight/cent titanium, but superconductor binary or higher alloys of the elements niobium, titanium, zirconium, hafnium and tantalum can be used. If required, substantially pure niobium can be used.

Other possible superconducting materials involve the use of a final heat treatment to produce the superconducting properties, and in this connection intermetallic compounds which have superconducting properties can be achieved. For example, the superconductor material can be a mixture or tin-rich alloy of niobium with tin, this being co-processed in the matrix For comparison, for oxygen-free high conductivity copper, to the final physical configuration required. The matrix can the resistivity at 20 C. is 1.7 micro-ohm-cm. and the Vickers comprise excess tin or niobium. A suitable heat treatment, for Hardness Numbers are 45 and 1 17 in the annealed and 60 perexample in the range 700 to 950 C. for 5 to 120 minutes, can cent cold-worked conditions respectively with respective ultithen be applied to produce the intermetallic compound Nb Sn 5 mate tensile Strengths f 14 and 24.

in a thickness not exceeding 0.005 cm. perpendicular to the The most Preferred alloys are the cupmmlckels, additions fi ld, Th upper i i f 950 C must be reduced if necessary thereto of small quantities of silicon, zinc and aluminum as so as not to exceed the melting point of the matrix. Nb Sn has well as manganese improving corrosion resis'ahce and slightly appreciably improved superconducting properties over those increasing electrical resistivities. Other preferred alloys are ofniobium,although it does suffer from brittleness. In this way P Q P f" bronzev which is a y ductile alloy, Evel'dure A, substantial lengths of the brittle material are achieved, and the whlch as ducnle as cupmmckel but cheaper and the copper manganese alloys Kutherm 41, which is less ductile than cupro-nickel but cheaper, and Kutherm I00 which is still less ductile but has a very high electrical resistivity.

l5 Alternative matrix materials are steels such as low carbon steel, nickel-chromium alloys, and titanium.

From FIG. 2, it can be seen that the electrical resistances of cupro-nickel range of alloys only reduce by about 2 microohm-cm. for a change in temperature from 20 C. to 4.2 K.

20 The resistivities at 4.2 K. are the ones which are relevant, and the actual values of these are given in Table II for the alloys specified therein, viz.:

cross-sectional dimensions of this brittle material can be arranged to be so small that the effects of brittleness are largely overcome, whereby reasonable deformation of the conductor can be achieved without breakage of the superconductor filaments.

DESCRIPTION OF THE DRAWINGS Typical examples of the invention will now be more fully described with reference to the accompanying drawings in which:

' FIG. 1 is a graph of the electrical resistance at 20 C. in micro-ohm-cm. for various copper-base binary alloys plotted against the percentage content of the indicated alloying metal; TABLE 1 FIG. 2 is a graph of the electrical resistance at 4.2 K. in micro-ohm-cm. for cupro-nickel alloys plotted against the percemage wi l content; Copper Resistivity in FIG. 3 IS a graph of the work-hardening curves (shown by (balance nickel) micro-ohm-cms the Vickers Hardness Numbers) during reduction in cross-sec- 88 (Pure copper) ia-3 2 tional area for the matrix alloys copper weight/cent nickel 30 80 23 and copper 20 weight/cent ntckel compared with the curves 70 38 for the superconductor alloy niobium 44 weight/cent titanium 6O 44 and oxygen free high conductivity copper;

FIG. 4 is a cross-sectional view of an assembly of supercondllciol' elements and cupro-nickelmatrix material; FIG. 3 shows the way in which the hardness of the two 5 is a Perspective View of a conductor with P of the copper alloys exemplified thereon changes during working. cupro-nickcl matfiX removed; and The abscissa of the graph of FIG. 3 gives a percentage reduc- FIGS- 6 and 7 are Current p Versus field tion in cross-sectional area during the working, while the orkilogauss) cur e o a ous twisted sup uc ors dinates give the hardness as the Vickers Hardness Number. dergoing changing fields. 40 Also plotted on this graph is the corresponding curve for the Referring initially to FIG. 1, the resistivities of various biu r du tor ll iobi 44 i ht/ i i d f nary alloys at 20 C. are plotted against the alloying metal conoxygen-free high conductivity copper. From this can be seen tent, the balance in each case being copper with incidental imthe great similarity in the work-hardening characteristics, parpurities. The resistivity curve for manganese additions extends ticularly at high reductions in area, between the cupro-nickel beyond the maximum value of resistivity appearing on the 45 range of alloys and the particular superconductor alloy con- Figure, and reaches 100 micro-ohm-cm. for the alloy copper cerned, h h h pplicable cupro-nickel alloy can be 30 weight/cent manganese. The most preferred alloys are 11 1 r h superconductor alloy. copper 5 5 weight/cent i k l 2 i h manganese, From the above, it can be seen that the required ductile the possible manganese addition having little effect upon elecpp can be SeleCted to match the physi trical resistivity but improving corrosion resistance. characteristics of the superconductor material to be used, and Various specific ductile alloys are listed in the following FIG. 1 and/or Table I then gives the resistivity at 20 C. to be Table I, the table including details of composition (balance expected from such an alloy which is very close to the resistivicopper), resistivity at 20 C. and 4.2 K., and hardnesses and ty at 4.2 K. It will be noted that even if the alloy 95 ultimate tensile strengths (U.T.S.) in the annealed and 60 perweight/cent copper balance nickel needs to be selected, this cent cold work conditions, viz: alloy will still exhibit a resistivity at 4.2 K. which is approxi- TABLE I Resistivity,

' micro-ohm-ums. Hardness U'IS (tons/sq. in.)

App. App. 60% Nature Ni Mn Sn Al Fl Si Zn Others 20 C. 4.2 K. Annealed uoltl work Annealed cold work (fmlstunl.m| l5 47 45 27 41 upmmitzlivl. 36/42 38 J0 182 24 3.) I m 27/30 23 no 168 22 30 l)o. 13/10 12 Phosphor Bronze 10 80 260 22 50 (Kuthvrm l0). Everduro A (Kutlmrm 26). Kutlivrm 41 Mnngnuin Nl(tl tl-SllVLl' Cu-MnNi Al-brmmcs.

Do Bmssvs (/30) Bi'uSSvS ((30/40) mately 700 times higher than the resistivity of 100 percent copper in zero field. Hence there will be an appreciable resistance in any closed current paths between adjacent superconductor filaments in a matrix of such an alloy, when compared to the situation when the matrix is high conductivity copper. However, high conductivity copper can be used, although the length L then needs to be small in accordance with the given relationship.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A first typical example of the invention will now be described. in this example a billet of the superconductor alloy niobium 44 weight/cent titanium is cast and then forged at about 600700 C. This is pickled in a mixture of nitric and hydrofluoric acids, and is quickly inserted in a prepared can of the alloy copper 25 weight/cent nickel. The can is prepared from a cast and forged billet, and is pickled in 50 percent nitric acid. In this typical example, the can has an external diameter of about 2% inches, although it is envisaged that 9 or 12 inches may be appropriate. The quantities of the superconductor alloy and the cupro-nickel are arranged such that there are about 6 parts of cupro-nickel to 10 parts of the superconductor alloy.

The can is evacuated and sealed to minimize any oxidation of the facing surfaces of the superconductor and the cupronickel, and is then extruded at about 350-650 C. with a reduction ratio of about 6:1. After extrusion, room temperature drawing is carried out with a total reduction of about 95 percent, the end result being an elongated hexagonal rod.

This rod is cut into, in this example, 61 lengths which are packed into a further can of the cupro-nickel alloy, any gross cavities being filled with further hexagonal rods of the cupronickel alloy. This assembly is shown in FIG. 4. The use of this can and the extra rods of cupro-nickel provide approximately equal quantities of the cupro-nickel and the superconductor alloy.

This assembly is evacuated and sealed and is again extruded at about 350-650 C. with a reduction ratio of about 6: 1. This is followed by a series of room temperature drawing to produce a reduction in cross-sectional area of at least 99.5 percent. This produces a wire which has an overall diameter of about 0.01 inch, containing the 61 filaments each of a diameter of about 0.001 inch. (If required, the room temperature drawing can be continued to provide filaments having a diameter of about 0.0005 inch or less.)

During the final draw the wire is twisted at a rate of one turn per inch, i.e. to give a length L 1 inch. The result is shown in FIG. in which part of the cupro-nickel matrix has been removed to show the filaments and their rate of twist. The overall conductor diameter is 0.01 inch, and the filament diameters are 0.001 inch.

The wire is then provided with a heat treatment for about 1 hour at 350-450 C. to refine the dislocation structure in the superconductor alloy, whereby the superconducting properties of this alloy are maximized, and this also serves to partially anneal the cupro-nickel alloy. If required, this heat treatment can also be applied during the room temperature drawing processes, typically when the wire has reached an overall diameter of about 0.025 inch. When this is carried out, the partial annealing of the cupro-nickel will facilitate the further co-processing.

Finally, the heat-treated wire is provided with an insulation coating as required, typically of polyvinyl acetate.

For this example, the formula given above can be used, the various parameters therein being k, the space factor, equal to 0.8; J, the low field current density for niobium 44 weight/cent titanium, as 3 X a filament diameter d of 2.5 X 10 cm.; a resistivity at 42 K. for copper 25 weight/cent nickel of p 3 X 10' ohm-cm; and a rate of change of field B 1,000 (1 kilogauss per second). Hence with L as one inch, which is 2.54 cm., a is about 66. This is well above the preferred value of a as 10.

The substitution of a as 10 in the formula to find the corresponding length L gives L 17 cm. approximately. Hence a twist pitch of 17 cm. should be quite satisfactory.

The substitution of a as 3 in the formula to find the corresponding length L gives L 56 cm. approximately. This should be marginally satisfactory.

If copper is used instead of cupro-nickel, taking its resistivity as 2 X 10* ohm-cm. and using the marginally satisfactory value for a as 3, the twist pitch L is 1.45 cm. Using the preferred value ofa as 10, the twist pitch L is only 4.4 mm. This may be too high a rate of twist to be performed satisfactorily and economically.

It will be noted that the compatibility between the alloys copper 25 weight/cent nickel and niobium 44 weight cent titanium enables reductions of this order without undue difficulty, and it is found that the filaments have a very uniform circular cross section, and are also uniform along their lengths. With filaments of these dimensions, any irregularity along their lengths would rapidly lead to breakage.

If required, transposing can be used instead of twisting in order that each filament shall be subjected to the same influence of magnetic flux. Thus a conductor containing large numbers of filaments will probably need to be transposed, the approximate criteria being that transposing is necessary if is greater l'ihan O delta B across conductor 0 mean of modulus B across conductor and mean of modulus along twist where f? is again the rate of change of field and delta is the variation in the rate of change of field, d,is the filament diameter and d is the conductor diameter.

In a second example of the invention an elongated hexagonal rod manufactured as described in the first example is cut into 18 lengths which are packed into a further can of copper 25 weight/cent nickel, any gross cavities being filled with further hexagonal rods of the cupro-nickel alloy. There results a matrix to superconductor ratio of about 2.26:1.

The assembly so produced is processed as described in the first example to produce a conductor having an overall diameter of about 0.025 cm. and a filament diameter d of 3.26 X 10 cm. The conductor was provided with a twist having a pitch length L of about cm. determined by the use of the formula with a 3, a space factor k as 0.75, and a rate of change of field B as 360 gauss/second.

This conductor was tested experimentally by being subjected to various currents, and for each current by being subjected to a transverse magnetic field rising at the rate of 360 gauss per second. The field value at which the conductor produced non-superconducting properties was then noted for each current, and the results plotted as the solid curve in FIG. 6. The solid curve indicates a satisfactory performance for the conductor.

However, when the conductor was subjected to a rate of change of field of 710 guass/second, which corresponds to a value for a of 2.64, the properties fell back to the dashed curve of FIG. 6. When the conductor was subjected to a rate of change of field of 1,400 gauss/second, which corresponds to a value for a of 1.88, the properties fell even further back to the very unsatisfactory dot-and-dash curve of FIG. 6.

In a third example of the invention a conductor was manufactured in the way described in the second example, except that its twist pitch was L 6 cm. When subjected to a rate of change of field E of 1,400 gauss/second, which corresponds to a 27 the dot-and-dash curve of FIG. 7 was produced. This is quite a satisfactory performance, and is better than that of the solid curve of FIG. 6. A rate of change of field of B 360 gauss/second, which corresponds to a 53, produced the marginal improvement shown by the solid curve of FIG. 7.

If required, the conductor can be provided with one or more strands of oxygen-free high conductivity copper to act as an electrical shunt in the event of gross break-down of superconductivity or breakage of the superconductor filaments. These strands can be embodied in the matrix or cabled with the conductor. When the matrix includes more than one non-superconductor material, it is the electrical resistance of the matrix material having the highest value thereof and located between the filaments which is to be used for p in the relationship given above because it is'the greatest electrical resistance which is effective in reducing magnetization currents.

This is of practical significance because, for many applications of these materials, where rapid changes of magnetic field are not required, the prescribed rate of twist is not imprac ticably high even when a low resistance matrix (for example copper) is used. In several important applications, however, (for example proton synchrotrons, and d.c. generators) it will be necessary to change the magnetic field at a rate typically in the region 1 kilogauss/second to 100 kilogauss/second; in these cases, to achieve a reasonable rate of twist it is necessary to use a matrix with a resistivity typically in the region 1 X 10 ohm-cm. (such as the appropriate alloys specified above). However, if the matrix is composed entirely of such a material it will be very difficult to protect any coil of appreciable size from burn-out in the event of an accidental transition to the non-superconducting state. Furthermore the low thermal conductivity of the matrix, together with the alternating current heat dissipation in the superconductor, may result in an undesirably large temperature rise in the superconductor.

These difficulties may be reduced by the use of a'matrix containing two materials, i.e. one of high resistivity to ensure a reasonable twist rate, and one of low resistivity (and high conductivity) to satisfy electrical protection and heat dissipation requirements. The geometric arrangement of the two components must be such that any electrical path connecting any two filaments must intersect a layer of the high resistivity material; for example one way of achieving this is for each superconducting filament to be first surrounded by a layer of high resistivity alloy, and for the resulting sheathed filaments to be set in a copper matrix. lnterchanging the two materials results in another valid possibility.

A simpler possibility is the use of a simple composite of superconducting filaments and high resistivity alloy surrounded by a concentric sheath of low resistivity metal (such as copper); however, although this would achieve electrical protection, it will not achieve such a high efiective thermal conductivity. Therefore the more intimate combination of the two matrix components as described previously, will be preferable in most cases.

- q The composite conductors described previously are, as stated, free from coil degradation arising from flux jumping. However, some degradation of performance can still arise from another cause namely the frictional heating resulting from movement of wire within the coil under the action of electromagnetic forces. To ensure the satisfactory performance of these conductors, therefore, it will usually be necessary to prevent or minimize such wire movement; one way of doing this is to impregnate the winding with, for example, an epoxy resin. In many cases, for example small coils, it will be sufficient to use a simpler impregnating material, such as paraffin wax.

We claim:

1. An electrical conductor in a changing magnetic .field comprising a plurality of filaments of a material which is superconductive at temperatures of about 4.2.K. and above contained within and separated from one another by a matrix of at least one material which is not superconductive at about 4.2 K., each filament having a maximum thickness of about 0.005 cm. and being twisted or transposed about the other filaments with a length L along the conductor between each return of that filament to a given angular position relative to the conductor given by the relationship:

where k is a space factor equal to the ratio between the linear dimensions of superconductor and those of superconductor plus matrix materials in a direction parallel to the magnetic field in which the conductor operates, J is the critical current density in amps/cm. of the superconductor material (in zero or very low magnetic fields), d is the average thickness in cms of each filament perpendicular tothe magnetic field, p is the electrical resistivity in ohm-cms of the non-superconductor material separating the filaments from each other and having the highest electrical resistance, 8 is the rate of change of field in gauss/second, and a is a number which is greater than 3 and is sufficiently large for the'variation in flux rate of change Q along the length L divided by the mean flux rate of change 8 along the length L to be less than the thickness :1 divided by the thickness of the conductor perpendicular to the field.

2. A conductor according to claim 1 wherein the number a is greater than 10.

3. A conductor according to claim 1 wherein the matrix is a ductile material.

4. A conductor according 'to claim 3 wherein the matrix includes a ductile copper-base alloy which contains not less than 50 weight/cent copper and has an electrical resistivity of at least 6 micro-ohm-cm. at 20 C.

5. A conductor according to claim 4 wherein the ductile copper-base alloy contains 5-50 weight/cent nickel, 0-2 weight/cent manganese, balance not less-than 50 weight/cent copper, and has an electrical resistance of at least 7 microohm-cm. at 4.2 K. 1

6. A conductor according to claim 1 wherein the superconductor material is selected from the group consisting of superconductingbinary or higher alloys of niobium, titanium, zirconium, hafnium and tantalum.

7. A conductor according to claim 6 wherein the superconductor material is niobium 44 weight/cent titanium.

8. A conductor according to claim 1 wherein-the superconductor material is the intermetallic compound Nb sn.

9. A conductor according to claim 1 wherein the conductor comprises a plurality of filaments of niobium 44 weight/cent titanium contained within a matrix of the ductile alloy copper 25 weight/cent nickel, k being 0.8, J being 3 X 10 d being 2.5

X 10', p being 3 X 10*, B being not more than 1,000, L being 1 inch and a being about 66 at least.

10. A conductor according to claim lwherein the conduc-. tor comprises a plurality of filaments or niobium 44 weight/cent titanium contained within a matrix of the ductile alloy copper 25 weight/cent nickel, k being 0.8, J being 3 X 10", d being 2.5 X 10, p being 3 X 10 B being not more than 1,000, L being 17 cms approximately and a being 10 at least.

11. A conductor according to claim 1 wherein the conductor comprises a plurality of filaments of niobium 44 weight/cent titanium contained within a matrix of the ductile alloy copper 25 weight/cent nickel, k being 0.8, J being 3 X 10*, d being 2.5 X 10 p being 3 X 10 B being not more than 1,000, L being 56 cm. approximately and a being 3 at least.

12. A conductor according to claim 1 wherein the conductor comprises a plurality of filaments of niobium 44 weight/cent titanium contained within a copper matrix, k

being 0.8, Jbeing 3 X 10 dbeing 2.5 X 10", p being 2 X 10',

B being not more than 1,000, L being 4.4 mm approximately and a being 10 at least.

14. A conductor according to claim 1 wherein the conductor comprises a plurality of filaments of niobium 44 weight/cent titanium contained within a matrix of the ductile alloy copper 25 weight/cent nickel, k being 9.75, I being 3 X 10", d being 3.26 X 10*, p being 3 X 10 B being not more than 360, L being 105 cm. approximately and a being 3 at least.

15. A conductor according to claim 1 wherein the conductor comprises a plurality of filaments of niobium 44 weight/cent titanium contained within a matrix of the ductile alloy copper 25 weight/cent nickel, k being 9.75, J being 3 X 10 d being 3.26 X p being 3 X 10", B being not more than 1,400, L being 6 cm. and a being about 27 at least.

16. A conductor according to claim 1 wherein the conductor comprises a plurality of filaments of niobium 44 weight/cent titanium contained within a matrix of the ductile alloy copper 25 weight/cent nickel, k being 9.75, J being 3 X 10 d being 3.26 X 10, p being 3 X 10', B being not more than 360, L being 6 cms and a being about 53 at least.

17. A conductor according to claim 1 wherein the matrix ineludes more than one non-superconductor material. 

1. An electrical conductor in a changing magnetic field comprising a plurality of filaments of a material which is superconductive at temperatures of about 4.2* K. and above contained within and separated from one another by a matrix of at least one material which is not superconductive at about 4.2* K., each filament having a maximum thickness of about 0.005 cm. and being twisted or transposed about the other filaments with a length L along the conductor between each return of that filament to a given angular position relative to the conductor given by the relationship: where k is a space factor equal to the ratio between the linear dimensions of superconductor and those of superconductor plus matrix materials in a direction parallel to the magnetic field in which the conductor operates, J is the critical current density in amps/cm.2 of the superconductor material (in zero or very low magnetic fields), d is the average thickness in cms of each filament perpendicular to the magnetic field, p is the electrical resistivity in ohm-cms of the non-superconductor material separating the filaments from each other and having the highest electrical resistance, B is the rate of change of field in gauss/second, and a is a number which is greater than 3 and is sufficiently large for the variation in flux rate of change B along the length L divided by the mean flux rate of change B along the length L to be less than the thickness d divided by the thickness of the conductor perpendicular to the field.
 2. A conductor according to claim 1 wherein the number a is greater than
 10. 3. A conductor according to claim 1 wherein the matrix is a ductile material.
 4. A conductor according to claim 3 wherein the matrix includes a ductile copper-base alloy which contains not less than 50 weight/cent copper and has an electrical resistivity of at least 6 micro-ohm-cm. at 20* C.
 5. A conductor according to claim 4 wherein the ductile copper-base alloy contains 5-50 weight/cent nickel, 0-2 weight/cent manganese, balance not less than 50 weight/cent copper, and has an electrical resistance of at least 7 micro-ohm-cm. at 4.2* K.
 6. A conductor according to claim 1 wherein the superconductor material is selected from the group consisting of superconducting binary or higher alloys of niobium, titanium, zirconium, hafnium and tantalum.
 7. A conductor according to claim 6 wherein the superconductor material is niobium 44 weight/cent titanium.
 8. A conductor according to claim 1 wherein the superconductor material is the intermetallic compound Nb3Sn.
 9. A conductor according to claim 1 wherein the conductor comprises a plurality of filaments of niobium 44 weight/cent titanium contained within a matrix of the ductile alloy copper 25 weiGht/cent nickel, k being 0.8, J being 3 X 105, d being 2.5 X 10 3, p being 3 X 10 5, B being not more than 1,000, L being 1 inch and a being about 66 at least.
 10. A conductor according to claim 1 wherein the conductor comprises a plurality of filaments of niobium 44 weight/cent titanium contained within a matrix of the ductile alloy copper 25 weight/cent nickel, k being 0.8, J being 3 X 105, d being 2.5 X 10 3, p being 3 X 10 5, B being not more than 1,000, L being 17 cms approximately and a being 10 at least.
 11. A conductor according to claim 1 wherein the conductor comprises a plurality of filaments of niobium 44 weight/cent titanium contained within a matrix of the ductile alloy copper 25 weight/cent nickel, k being 0.8, J being 3 X 105, d being 2.5 X 10 3, p being 3 X 10 5, B being not more than 1,000, L being 56 cm. approximately and a being 3 at least.
 12. A conductor according to claim 1 wherein the conductor comprises a plurality of filaments of niobium 44 weight/cent titanium contained within a copper matrix, k being 0.8, J being 3 X 105, d being 2.5 X 10 3, p being 2 X 10 8, B being not more than 1,000, L being 1.45 cms approximately and a being 3 at least.
 13. A conductor according to claim 1 wherein the conductor comprises a plurality of filaments of niobium 44 weight/cent titanium contained within a copper matrix, k being 0.8, J being 3 X 105, d being 2.5 X 10 3, p being 2 X 10 8, B being not more than 1,000, L being 4.4 mm approximately and a being 10 at least.
 14. A conductor according to claim 1 wherein the conductor comprises a plurality of filaments of niobium 44 weight/cent titanium contained within a matrix of the ductile alloy copper 25 weight/cent nickel, k being 0.75, J being 3 X 105, d being 3.26 X 10 3, p being 3 X 10 5, B being not more than 360, L being 105 cm. approximately and a being 3 at least.
 15. A conductor according to claim 1 wherein the conductor comprises a plurality of filaments of niobium 44 weight/cent titanium contained within a matrix of the ductile alloy copper 25 weight/cent nickel, k being 0.75, J being 3 X 105, d being 3.26 X 10 3, p being 3 X 10 5, B being not more than 1,400, L being 6 cm. and a being about 27 at least.
 16. A conductor according to claim 1 wherein the conductor comprises a plurality of filaments of niobium 44 weight/cent titanium contained within a matrix of the ductile alloy copper 25 weight/cent nickel, k being 0.75, J being 3 X 105, d being 3.26 X 10 3, p being 3 X 10 5, B being not more than 360, L being 6 cms and a being about 53 at least.
 17. A conductor according to claim 1 wherein the matrix includes more than one non-superconductor material. 